Electrical burglar-alarm.



A. ZEHDEN.

ELECTRICAL BURGLAR ALARM.

APPLICATION man MAR. 18. I914.

1,257,440. Patented Feb.26,191&

i l l l I- b i'inesses: flare/z f0r ALFRED ZEHDEN, OF BERLIN, GERMANY.

ELECTRICAL BURGLAR-ALARM.

Specification of Letters Patent. Patented Feb, 26, 1918,

Application filed March 18, 1914. Serial No. 825,637.

To all whom it may concern:

Be it known that I, Dr. ALFRED ZEHDEN, a subject of the Emperor of Germany, and residing at Berlin, Germany, have invented certain new and useful Improvements in Electrical Burglar-Alarms, of which the following is a specification.

In electrical burglar alarms, the alarm signal is usually caused by the fact that, on the slightest shaking of the safe or the like which has to be protected, the alarm circuit is indirectly or directly closed by an oscillating or vibrating contact. Since thiscontact device normally maintains the alarm current circuit open and the conductors leading to same could therefore be cut without having any effect on the alarm signal, a resistance must be arranged in connection with the apparatus, which, at the given voltage, allows the required working current to flow through the conductors leading to the alarm, and which becomes short-circuited when the contact apparatus is shaken. It is, however, possible by means of a known artifice or trick to replace any resistance by a combination of auxiliary resistances and in this manner to render the installation inoperative, as shown in Figure 1. For the sake of simplicity all contact and alarm apparatus and the like are omitted. The source of current e is connected with the resistance 'w by means of leads a and I), said resistance being usually connected up as the branch of a Wheatstone bridge for the purpose of increasing the sensitiveness of the installation to changes in the current. To effect the trick or artifice one requires only two large sliding resistances 18, 19. A wire is connected at any desired point 22 to one of the leads a.- This wire is connected to the two sliding contacts 20, 21. The resistance 18 is connected to a point of the lead 6 and the contact 20 is set at its other end, so that the practically infinitely great resistance is in parallel with the alarm resistance w, whereby alterations in the current are not brought about. One of the ends of the resistance 19 is connected to the lead a near point 22 and the contact 21 is set at this end,

whereupon this lead is out between points 21 and 22. These positions of the contacts 20 and 21 are shown in Fig. 1. bince the series resistance 19 is infinitely small, it does not cause any change in the current. If now the auxiliary resistance 19 is gradually in creased, until it becomes infinitely great, and

if at the same time the auxiliary resistance 18 is gradually decreased, until it becomes of the same value as the resistance to, then the current flowing through w will indeed be infinitely small, but the strength of the current flowing in the leads a and 1) between the source of current and the tappings will not be changed, so that the alarm devices in the watch-room do not operate.

If now one knocks against the vibrating device and in this manner possibly shortcircuits w, or if one cuts through the leads in the neighborhood of w, this will not be indicated in the watch-room, since the infinitely great resistance 19 prevents practically measurable changes in the current. The working resistance w is thus, by means of this artifice, substituted by the combination of a parallel resistance 18 with a series resistance 19, and in this manner it is possible to gain apcess to the protected point without the protective installation indicating this. With some practice, one can in this manner, especially if'one connects up a sensitive current measuring instrument or ammeter in one of the leads a or Z), render the known arrangements of protective installations inoperative in a few minutes, regardles's of the manner in which they are individually connected up.

The present invention removes this drawback in that the ordinary resistance is replaced by a branched resistance, and in that the latter is connected to more than two voltages. In -the new method of connection several circuits are thus employed, which are interconnected with one another throughthe compound resistance and preferably besides this through the source of current. If now by use of the artifice, the current of the partof the compound resistance lying between two leads is weakened the voltage condi'-- tions in the latter are altered, and therefore also the current strength and in general the current distribution of the other circuits, which can be utilized in any desired and in itself known manner for bringing about the action of the alarm signal. Whereas therefore when the artifice is applied between any two leads the current in same in the watch- I room remains unaltered, it necessarily becomes altered in the other leads, so that the artifice does not act with the new system of connections. This holds good for direct and alternating currents.

In Fig. 1 of the drawing, the principle of the system-of connections is shown diagrammatically, the signal and contact devices being omitted. Figs.'2, 3 and 4 show forms of burglar alarm installation based on this principle, the watch-room being on the left and the point of danger on the right. The.

leads of the alarm current circuits are shown dotted.

As shown in the diagram Fig. 1, the resistance w consists of four individual bridge resistances 'w 10 w, to. Two opposite corners of the bridge are connected by means of the leads a and cl with the terminals 1 and 2 of the battery e. The two other corners of the resistance bridge could be connected by means of the leads I) and c with another source of current, but they are preferably connected with points 3 and 4 of the sub-divided battery, so that the currents in the alarm leads are coupled together both 7 through the battery and through the resistance to, the latter being connected to four diiferent voltages. The changes in the current occurring between two leads due to. the application of the artifice, or to any current changes within the installation or to switchmg in or out of resistances or of electromotive forces could be made discernible in the other leads in any desired manner, for example, in an acoustic manner by means of an instrument or relay in three of the leads a, b, c, d, d being regarded as a common return lead.

When using this principle for the alarm installation according to Fig. 2, only one relay is required which is connected up as a current measuring instrument of a Wheat-.

. the twoother corners of the resistance to.

The latter therefore is normally connected to three points of difierent voltages and only when the equilibrium of the. bridge is disturbed is it connected to four points of different voltages. The alarm current circuit con- Sists'of an alarm bell orgong h, the connecting leads k and l to the battery and to the contacts 112. which are closed by the relay g. Preferably the bridge is in itself not balanced and the resistances f f are of various values which are so calculated that the neutral lead of the bridge has no current flowing through it only when the resistance w is connected in parallel in the manner shown. If the leads a, b, c, d are maliciously cut during the day, the alarm signals under normal conditions.

will sound as soon as the installation is switched in.

To complete the system of connections there still remain to be mentioned the switch 9 drop when a-minimum current limit is reached, whereby a mechanical alarm 1' is released and set into action.

In the example of the system shown in,

Fig. 3, the four leads are laid separately in pairs, said two pairs being spaced apart in order to render more diflicult a simultaneous cutting of same. They lead to two separate compound resistances to which are arranged either at two different safes or say at a safe and at the door of the strong-room, and which are each connected with the contact apparatus 0 and with each other by means of two leads 8 and t. If the voltages and the resistances are correctly chosen, these leads have current flowing through them The resistance in this case is not in the form, of a bridge, but is in the form of a triangle, the middle point of which is connected with the three corners. The four alarm leads are connected totwo corners of each of. the resistances, the two third corners and the middle points of the two triangles bein connected by means of the said leads t an s.

The switching in of the alarm circuit is effected by two difierential relays. The two coils of each relay lie in alarm leads which are spaced apart from one. another; thus the coils u a are in the leads a and I), while the two coils u u of the other relay are in the leads 0 and d. The alarm current circuit again consists of the gong h, of the connecting leads '0, 20 and av, between the gong and the battery and of the contacts m, m which are short-circuited by the relay. If the proportion of the currents in two leads varies, then one or the other relay short-circuits the alarm current circuit. When the 4-po1e batteryswitch w, m, m, w is opened, the armatures of the two relaysdrop and therefore short-circuit the two sets of contacts m, m and m m Thus the alarm signals will at first always sound when the switch is closed. If the system of connections is in order, then the armatures will quickly assume their normal neutral position and the alarm will stop if on the other hand, all 4 leads were cut throu h when the battery was switched out, then t e alarm signal will continue sounding if the four relay coils and the resistance to are correctly chosen.

For the completion of the installation two door contacts are introduced between the leads 6 and c and one door contact between the leads a and 03. Each of these contacts consists of the actual contact lever y and of a resistance 2 and of the connecting leads. When the door is opened, the resistance a is switched out and in this manner the relay is brought into action, thus closing the alarm current circuit.

The reliability of the installations according to Figs. 2 and 3 is further increased if the resistances w are not dimensioned in the usual manner but are designed as hotwire resistances. If in this case the above mentioned artifice is employed, the changes in current in the individual branches of the resistance will 'alter the value of its ohmic resistance, whereby the disturbance of the equilibrium is strengthened. If the temperature of all the branches of the resistance w varies uniformly, the equilibrium will not be disturbed, so that variations of the surrounding temperature will have no efi'ect on either of the examples of the system.

In the example shown in Fig. 4 it is assumed that the bridge resistance w is of the hot-wire type. It is connected as the branch of a bridge, so that the installation represents a double bridge of great sensitiveness. The battery e has its two terminals 1, 2 connected to the two resistances f of the bridge, the third resistance of which is F,

and the fourth resistance of which is represented by the bridge resistance w situated at the point of danger. The connecting leads between the bridge and two oppositely disposed corners of the resistance w are again denoted by a and d. The two other corners of the resistance could again be connected by means of leads 6 and c with points of the sub-divided battery 6. According to the example of the system (Fig. 4) they lead, however, to another source of current 0 which could be a source of alternating current but has been assumed -to be a. battery. In the lead I) there is a current measuring instrument or relay 9 which operates with direct or alternating currents and which closes the alarm current circuit of the gong h, as does also the direct current measuring instrument 9 of the bridge, when the equilibrium is disturbed. The four branches of the resistance to are preferably of different values in order that the circuits of the two sources of current influence one another mutually. Owing to the sensitiveness of the system, the current from the battery e need only be extraordinarily small. As will be seen the alarm current circuit will be closed whenever there is' a disturbance by either one or the other instrument g, 9 but generally, however, by both simultaneously. The infiuence of the surrounding temperature can be removed by having in the branch f of the resistance a hot-wire resistance which is congruent with the resistance to and which, if necessary, is installed at the point of danger.

For the completion of the system the following parts are introduced in the diagram Fig. 4. A double-pole switch 5 serves for the simultaneous switching in or out of the two sources of current. Before each of the four corners of the resistance to a polarizing cell 6 is introduced in the circuit. When making use of the above mentioned artifice, the current is therefore suddenly interrupted when the auxiliary and series resistance 19 according to Fig. 1 reaches a certain value, namely when the voltage at the terminals of the cell sinks below a certain value. In this manner a sudden interruption ofthe current 1s attained instead of the desired gradual cutting out of the resistance 'w. Further an electrical sound receiver 7, that is, a telephone or microphone is connected in parallel to the resistance 'w at the point of danger in the circuit of the battery 6 On account of the fact that the resistance of a microphone does not remain constant and in order to avoid loss. of current through the sound receiver during normal service, this apparatus 7 is placed in series with a condenser 8. Thus as soon as the burglar approaches the point of danger, that is, when the doors of theroom are broken open, or when the burglar is making use of his house-breaking tools, the sound of steps and even of speech conducted in a low tone, will by reason of the acoustic vibrations cause the sound receiver 7 to send im ulses of current in known manner not only to the relay 9 but also to the hot-wire resistance w; in this manner the value of same is altered and thereby the relay 9 is put into operation. Further the leads a and d and the lead I) are coupled inductively with one another through the series coils 9, so that sudden current variations also in the two first mentioned leads, for instance, when the current circuit is interrupted bythe polarizing cell 6 pertaining to same when an auxiliary series resistance is being gradually increased, will set the alarm signal 1n operation by means of the instrument 9.

The new alarm system is also suitable for connecting to three-phase mains, since from the nature of three-phase systems more than two conductors having different voltages are present.

Having now described my invention what I claim as new and desire to secure by Letters Patent is 1. In an electrical burglar alarm, the

combination in the alarm circuit at the point of danger of a branched resistance and connections therewith to more than two points of different potential in the source of current.

2. In an electrical burglar alarm, the combination in the alarm circuit at the point of danger of a branched hot-wire resistance and connections therewith-to more than two points of diiferent potential in the source of current.

3. In an electrical burglar alarm, the combination ofta branched resistance, a measuring bridge, a current measuring instrument connected thereto, and an alarm signal, the latter being operated by the said current measuring instrument when a disturbance of the equilibrium occurs.

"as a potential divider and the resistance is normally connected to three points each having a difierent potential, and is connected to four such points when the equilibrium is disturbed. v

5. In an electrical burglar alarm, the combination of a branched resistance in bridge form, a measuring bridge unbalanced in itself and a source of current, two corners of the branched resistance bridge being connected in parallel with two corners of the measuring bridge to the terminals of the source of current, the second two corners of the resistance bridge being connected to sec-1 0nd two corners of the measuring bridge so that equilibrium is only obtained when the branched resistance is connected in parallel with the measuring bridge.

6. In an electrical burglar alarm, thecombination of a hot-wire resistance inobridge form which represents a branch of a measur= ing bridge, twoalarm leads connecting its corners to the measuring bridge and to the source of current and the two other corners being connected with difi'erent potential points of the source of current in the bridge.

7. In an electrical bur lar alarm, the combination of a branched resistance, alarm leads and a source of current, the alarm leads being connected to at least three points of different potentials in the source of current, for the purpose of providing an increased mutual influence of the individual alarm .leads by means of each of the'alarm leads being connected in multiple through the resistance and through the source of current. H

8. In a burglar alarm, a source of current, a branched resistance, connections between the branched resistance and more than two points of different potentials of the source of current which are transversed in part continuously and in part only upon disturbance by currents; a minimum relay in one of the connectors continuously transversed by current and a mechanical alarm attachment which is' released by the armature of the relay upon falling below a current value.

In testimony .whereof I have signed my name to this specification in the presence of two subscribing witnesses.

DR. ALFRED ZEHDEN.

Witnesses:

HENRY HASPER, Womnmn HAUr'r. 

